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Experimental study on the compound system of proanthocyanidin and polyethylene glycol to prevent coal spontaneous combustion
Fuel 254 (2019) 115610
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Full Length Article
Experimental study on the compound system of proanthocyanidin and polyethylene glycol to prevent coal spontaneous combustion
T
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Zhian Huanga,b,c, Xiaohan Liua, Yukun Gaoa, , Yinghua Zhanga, Ziyou Lia, Hui Wanga, Xinrui Shia a
State Key Laboratory of High-Efficient Mining and Safety of Metal Mines (University of Science and Technology Beijing), Ministry of Education, Beijing 100083, China Work Safety Key Lab on Prevention and Control of Gas and Roof Disasters for Southern Coal Mines (Hunan University of Science and Technology), Xiangtan 411201, China c State Key Laboratory Cultivation Base for Gas Geology and Gas Control (Henan Polytechnic University), Jiaozuo 454000, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Coal spontaneous combustion Programmed temperature Chemical inhibitor Electron spin resonance Infrared spectrum
In order to overcome the short inhibition duration of physical inhibitor, the environmentally friendly chemical inhibitors of polyethylene glycol (PEG) and proanthocyanidin (OPC) were selected to conduct the experimental study for coal spontaneous combustion prevention. Through temperature programmed experiment, electron spin resonance experiment and Fourier infrared spectroscopy experiment, it was found that for the coal sample after treated by PEG and OPC, crossing point temperature increased by 17.6 °C and 14.3 °C, free radical concentration decreased to 0.1512 Ng/1017 g−1 and 0.1178 Ng/1017 g−1, association and hydroxyl free hydroxyl groups decreased by 59.72% and 24.37%, and the stability of the CeOeC ether bond increased by 42.71% and 18.41%, respectively, which showed the inhibition effects of both inhibitors are obvious and OPC is better. For the sample treated by OPC and PEG (ratio 1:2), crossing point temperature increased 25 °C, which indicated that the collaborative effect was remarkable. The analysis shows that PEG and OPC contain a large number of hydroxyl groups, which react with the oxide intermediate product alcohol to form ether bonds with relatively stable chemical properties, thus mitigating the rate of coal spontaneous combustion; PEG and OPC are linked by hydrogen bonds to synthesize larger hydrogen proton donors, which improved antioxidant activity. PEG is a viscous liquid, when carrying proanthocyanidin, can cover the surface of coal, isolating coal surface from oxygen, keeping moisture and cooling. The combination of the two inhibitors has significant synergistic inhibition effect, which can greatly suppress the coal spontaneous combustion.
1. Introduction Coal industry plays an important role in economic development. Unfortunately, fires caused by spontaneous combustion are a worldwide problem for the coal industry [1–5]. For the suppression of spontaneous combustion of coal, the inhibitor fire prevention technology has been widely applied, obtaining good results. Based on recent scientific research results, the main inhibitors for coal spontaneous combustion are the following: (1) halogen salt and ammonium salt inhibitor [6–8]; (2) gel inhibitor [9–11]; (3) compound inhibitor [12–14]; (4) polymer inhibitor [15–19]; and (5) foam inhibitor [20,21]. The inhibiting mechanism of this kind of inhibitors is mainly physical inhibition, which has disadvantages such as low inhibition efficiency and short inhibition life. However, chemical inhibitors prevent spontaneous combustion of coal by destroying or reducing the structure with higher oxidation reactivity or eliminating free radicals in coal in
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advance. Compared with physical inhibitors, chemical inhibitors have a longer inhibition period [22]. The research for the chemical inhibitors is currently at the initial stage, mainly some antioxidants [23] and metal compounds [22]. The former can control oxide free radical chain reaction, such as naphthylamine, naphthol, diphenylamine three phenol, hydroquinone and PCB, and the latter includes perchlorate, potassium permanganate, sodium persulfate, etc. Research results are relatively less, especially the environmental performance, inhibition characteristics and inhibition mechanism needs to be improved, and further research. In this study, the environment-friendly proanthocyanidin (OPC) and polyethylene glycol (PEG) were selected to investigate inhibiting properties and inhibiting mechanism. Proanthocyanidins (OPC), also known as condensed tannins, are polyphenolic compound, widely contained in the nucleus, skin or seeds of plants and they are among the most abundant polyphenols in our diet. Besides their participation in food quality attributes such as
Corresponding author. E-mail address: [email protected] (Y. Gao).
https://doi.org/10.1016/j.fuel.2019.06.018 Received 3 February 2019; Received in revised form 2 May 2019; Accepted 5 June 2019 Available online 18 June 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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astringency, bitterness, aroma and color formation, proanthocyanidin consumption has been associated with numerous health benefits due to their antioxidant, anti-carcinogenic, cardioprotective, antimicrobial and neuro-protective activities. As a result, they are considered as functional ingredients in botanical and nutritional supplements [24]. And it comes from a wide variety of source and is easy to obtain. In addition, proanthocyanidins are powdery at room temperature and easy to transport. At the same time, OPCs have strong antioxidant and free radical scavenging activities. Many phenolic hydroxyl groups in proanthocyanidins release H+ to competitively combine with free radicals and oxides, so as to block free radical chain reactions. Experiments have shown that the scavenging activity of proanthocyanidins and their metabolites was generally stronger than that of Vitamin C and Vitamin E [25]. Polyethylene glycol (PEG) is a linear polyether composed of several ethoxy repeating units. PEG has a narrow molecular weight distribution and can be dissolved in aqueous solutions and other organic solvents. Due to its unique physical and chemical properties, polyethylene glycol has been widely used [26–28]. At room temperature, PEG-200 is a colorless, odorless and viscous liquid, soluble in water, ethanol and many other organic solvents. PEG-200 has good thermal stability, lubricity, moisture retention and dispersion. It is not easy to hydrolyze, and nontoxic and nonirritating. In addition, PEG-200 is widely used in industry as moisturizer, softener, antistatic agent, etc. The hydroxyl groups in the polyethylene glycol structure can etherify with the hydroxyl groups on the coal surface, and the ether bond is relatively stable in the coal oxidation process. Therefore, polyethylene glycol can remove the active hydroxyl functional groups on the coal surface. Moreover, PEG is stable in highly oxidizing conditions, such as in acidic, alkali, high temperature, and oxygen and hydrogen peroxide rich environments, and has low flammability and good biodegradability. Based on the theory of free radical chain reaction of coal spontaneous combustion, lignite was used as the subject of this study, and proanthocyanidin and polyethylene glycol were selected as the environmentally friendly inhibitors to carry out the experimental investigation of the chemical inhibitor mechanism of coal spontaneous combustion. The temperature programmed system was used to simulate the conditions and environment of low-temperature oxidation spontaneous combustion of coal [29,30], and the index gases produced by coal oxidation were analyzed by gas chromatography. The changes of the free radicals in the coal before and after the inhibition were analyzed by electron spin resonance (ESR) spectroscopy, and the changes of the number of active functional groups in the molecular structure of the coal after inhibition were qualitatively and quantitatively analyzed by infrared spectroscopy. In this study, the single inhibitory effect and the compound inhibitory effect of proanthocyanidin and polyethylene glycol were studied, the optimal compound was determined, and the inhibitory mechanism was analyzed.
Fig. 1. Three prepared coal samples.
absorption of polyethylene to make it dry naturally at room temperature for 48 h. According to the literature [32], when the amount of OPC and PEG200 is 10%, the inhibiting effect is the best. Therefore, coal (1 kg) was weighed into a beaker and 10% proanthocyanidins by weight (OPC, 100 g) was added, mixed well and let stand for 24 h to obtain the coal sample. Coal (1 kg) was weighed into a beaker and 10% polyethylene glycol by weight (PEG-200, 100 g) was added, mixed well and let stand for 24 h to obtain another coal sample. The three prepared coal samples are shown in Fig. 1. 2.2. Experimental study of inhibition by a single inhibitor 2.2.1. Programmed temperature and oxidation experiment of coal spontaneous combustion Low-temperature oxidation of coal is a necessary stage of coal spontaneous combustion, and the study of this stage can explore the direct cause of coal spontaneous combustion. In this study, XCT-0 temperature programmed system was used to simulate the conditions and environment of low-temperature oxidation spontaneous combustion of coal, and the temperature programmed experiment was conducted on raw coal and coal samples treated by inhibitors to study the inhibiting effect. In this experiment, the dosage was 1 kg for each coal sample. Air bottle was used to supply air, and the control gas flow was 100 ml/min, the gas pressure was 0.1 MPa. The temperature rise ranged from 30 °C to 130 °C, the heating rate was 0.5 °C /min. The end of the air outlet tube was connected with a high temperature resistant hose, and this hose was connected with a 500 ml gas sampling bag, and the generated gas was sampled every 10 °C for a total of 11 times. Then the gas was extracted with a medical syringe and injected into an Agilent 7820A high-precision gas chromatographic analyzer to analyze the gas composition and concentration. The central temperature of the coal sample was monitored by thermocouple to record the temperature changes during the experiment.
2. Experiment
2.2.2. Experimental study of free radical of the coal samples by ESR A mixture of dried coal samples with the diameter of 0.2 mm or less was selected, and the dosage of 2 mg was used for each experiment. A JES-FA200 EER Spectrometer (Japan) electron spin resonance spectrometer with the microwave frequency of 9041.47 MHz, microwave power of 1 mW, central center magnetic field of 323.144 mT, scanning width of 5 mT, modulation frequency of 100 kHz, modulation width of 0.01 mT, time constant of 0.03 s, scanning time of 1 min and magnification of 100 times was used. The experiments were conducted at room temperature.
2.1. Preparation of experimental materials The preparation procedures of the coal samples included crushing, screening, mixing, shrinkage and air drying carried out according to GB474-2008 “Preparation method of Coal Samples”, the safety production industry standard of the People's Republic of China [31]. Brown coal with the typical ignition point of 267–300 °C in the Hailar Baozhileer Mine, Inner Mongolia was used for the experiment. A jaw crusher was used to crush the coal samples. The coal samples with the particle sizes of 0–0.9 mm, 0.9–3 mm, 3–5 mm, 5–7 mm and 7–10 mm were sifted using a vibrating screen. The coal samples with different particle sizes were weighed to obtain 0.2 kg of coal for each sample. After fully mixing and separating, it was put into the sealed bag made of polyethylene to isolate oxygen and make use of the water
2.2.3. Fourier infrared spectroscopy experiment of the coal samples A mixed coal sample with the particle size of 0.2 mm or less were selected, and completely cover the ruby in the center of the platform for 2
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each experiment. A VERTEX 70 Fourier infrared spectrometer was used. The scanning wavenumber range was 600–4000 cm−1, the resolution was 4 cm−1, the scanning time of the sample was 2 min, and the background scanning time was 2 min. The experiments were conducted at room temperature. 2.3. Experiment investigation of the compound inhibitor OPC are easy to oxidize, and PEG is in the state of viscous liquid naturally. When the two are combined, PEG can play the role of covering and wrapping the coal surface, isolating oxygen, and can play a better role with OPC. A raw coal sample (1 kg) was placed into a beaker and 10% by weight of the compound inhibitor was added. The proportions of proanthocyanidins to polyethylene glycol were 1:1, 1:2, 1:4 and 1:8. The composite inhibitor was mixed with the raw coal sample uniformly and completely, placed into a sealed bag and stored for 24 h. The programmed temperature and oxidation system used to study the compound inhibitor was the same as that used for the inhibited coal sample as described above.
Fig. 3. Variation of the furnace temperature and center of the coal temperature with time for the OPC-inhibited coal sample.
3. Results and discussion 3.1. Experimental study of inhibition by a single inhibitor 3.1.1. Programmed oxidation and heating test of coal spontaneous combustion (1) crossing-point temperature It can be observed from the results of the programmed oxidation and heating test of raw coal that as the temperature increases, the crossing-point temperature of the raw coal sample appears at approximately 120 min (Fig. 2); in other words, the crossing-point temperature of the raw coal is 145.2 °C. After the addition of OPC as a retarding agent, the cross-point temperature of the raw coal appears at approximately 130 min (Fig. 3), effectively delaying the time of the appearance of the crossing-point temperature of the coal sample. The crossing-point temperature of the coal sample treated with inhibitors is 162.8 °C, which is 17.6 °C higher than that of the raw coal. It can be inferred that OPC plays a certain role in inhibiting the oxidation reaction of lignite and reducing the heat accumulation process. From the programmed temperature experiment on the raw coal, it was observed that the temperature of the center of the raw coal sample and the furnace temperature reached the maximum temperature
Fig. 4. Variation of the furnace temperature and center of the coal temperature with time for the PEG-inhibited coal sample.
difference at approximately 80 min. When polyethylene glycol was added as the inhibitor (Fig. 4), the temperature difference between the central temperature and the furnace temperature of the coal sample reaches the maximum at 100 °C, which is greater than the corresponding temperature difference of the raw coal sample. The crossingpoint temperature of the coal sample treated with inhibitors is 159.5 °C, which is higher than that of the raw coal sample by 14.3 °C (Fig. 5), indicating that polyethylene glycol plays the role of an inhibitor in the oxidation process of lignite and reduces the tendency for coal spontaneous combustion. (2) CO and CO2 gas concentration analysis Figs. 6 and 7 show the curves of the CO and CO2 concentrations with respect to the temperature obtained by programmed oxidation and temperature experiment of proanthocyanidin-inhibited coal samples and polyethylene glycol-inhibited coal samples. Fig. 6 shows that carbon monoxide concentration increases with increasing temperature. The carbon monoxide concentration curve of the coal treated with OPC and polyethylene glycol is always below that of the raw coal. Starting at 100 °C, the difference between the curves of the proanthocyanidin-inhibited coal and raw coal gradually increases, and the inhibition effect of the OPC on the oxidation reaction of the coal is more obvious. A distinct difference is observed between the curves for the polyethylene
Fig. 2. Furnace temperature and center of the coal temperature as the functions of time in the programmed temperature test of raw coal. 3
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Fig. 5. The intersecting temperature of three coal samples.
Fig. 8. Oxygen consumption rates of the OPC- and PEG-inhibited coal samples with temperature.
during the oxidation process. The carbon dioxide concentration curves of the coals treated with OPC and polyethylene glycol are always below the carbon dioxide concentration curves of the raw coal samples. When the temperature rises to 100 °C, the difference between the curves of OPC and raw coal increases gradually, and the difference between the curves of polyethylene glycol and raw coal clearly increases. It was concluded that the inhibition effect of polyethylene glycol is good, but the inhibition effect of OPC is more obvious. (3) Oxygen consumption rate The oxygen consumption rates of proanthocyanidins- and polyethylene-glycol-inhibited coal samples for different temperatures were obtained by the programmed temperature test (Fig. 8). It is observed from Fig. 8 that the oxygen consumption rate of the coal samples clearly decreases after the OPC and polyethylene glycol treatment at the same temperature, and the curve for the oxygen consumption rate as a function of temperature is relatively gentle, reducing the oxygen consumption rate of the coal sample and inhibiting the oxidation reaction.
Fig. 6. CO concentrations of OPC- and PEG-inhibited coal samples plotted versus the temperature.
3.1.2. Experimental study of free radicals of coal samples by ESR Fig. 9 shows the ESR spectra of the raw coal sample and coal samples treated by OPC and polyethylene glycol obtained in electron spin resonance experiments. The abscissa is the intensity of the magnetic field, and the ordinate is the strength of the primary differential curve of the absorption line. The main parameters of the ESR spectrum
Fig. 7. CO2 concentration of OPC- and PEG-inhibited coal samples plotted versus temperature.
glycol-inhibited coal and raw coal, and the amount of carbon monoxide reduction is clearly observed. Thus, it was found that polyethylene glycol inhibits the oxidation reaction of coal to some extent. As shown in Fig. 7, the concentration of carbon dioxide increases
Fig. 9. ESR spectra of the coal samples. 4
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Table 1 Main parameters of the ESR spectra of the coal samples. Coal sample
Free radical concentration Ng/1017 g−1
Radical concentration increment Ng/1017 g−1
g value
Line width △H/mT
Raw coal Add PEG Add OPC
7.8332 7.7154 7.6820
– −0.1178 −0.1512
2.00263 2.00261 2.00259
596.2 596.8 598.3
are shown in Table 1. (1) Free radical concentration Ng According to the free radical chain reaction theory, free radicals are produced and consumed continuously in the interaction of the coal and oxygen. If the rate of free radical generation is higher than the rate of consumption, the free radical concentration will increase, otherwise, it will decrease. The obtained experimental data show that the total concentration of free radicals in the coal decreases with the addition of inhibitors. It is inferred that PEG and OPG inhibitors can effectively remove active free radicals in the coal, and accelerate the termination rate of the free radical chain reaction, thus suppressing the oxidation activity of the coal, inhibiting the free radical reaction between the coal and oxygen, and achieving the goal of preventing coal spontaneous combustion. Based on the obtained values for the reduction in the free radical concentration, the inhibition effect of OPC on the free radical reaction is stronger than that of polyethylene glycol.
Fig. 10. Infrared spectrum of three coal samples.
An examination of the infrared spectroscopy data shows that the infrared spectroscopy absorption peaks of the coal samples are mainly distributed in the vibration frequency region of aromatic hydrocarbons; the high content of aromatic hydrocarbons ensures the stability of the molecular structure of the coal. The infrared absorption peaks that affect the oxidation performance of the coal samples are mainly distributed in the vibration frequency range of fat-based and oxygencontaining functional groups. After treatment with the inhibitor, the absorption peak quantities decreased in the infrared absorption region that affects the oxidation performance of the coal samples.
(2) g factor value
(2) Quantitative analysis of the infrared spectra
The g factor represents the number of free radical species in coal samples. A larger g value indicates that more different free radicals are present in the coal samples. It is observed from the molecular structure of coal that the unpaired electrons in the coal are more localized at the C, H, O, N, P, and S atoms and that the coal may also contain metal complex radicals. An examination of the data presented in Table 1 shows that the obtained g factor values are in the order of the raw coal sample > the polyethylene glycol resistive coal sample > the proanthocyanidin gas resistive coal sample. It is inferred that the active radicals in the inhibitor are combined with the active radicals in the raw coal, reducing the total content of different radicals.
The infrared spectra were quantitatively analyzed by setting the spectral baseline and calculating the area surrounded by the characteristic absorption peak and the baseline to determine the amount of the corresponding functional groups. To further determine the changes in the functional group structure of the coal sample treated with inhibitors and the original coal sample, the characteristic peak areas of the infrared spectra of the coal samples were calculated using the ORIGIN software. The results are shown in Table 2. The functional group changes of the inhibited coal sample relative to the original coal sample are shown in Table 3. An examination of the data presented in Tables 2 and 3 shows that many free hydroxyl and associative hydroxyl groups are present in raw coal, whereas the content of free hydroxyl and associative hydroxyl groups in the coal treated with the inhibitor is clearly reduced. The content of the free hydroxyl and associative hydroxyl in the proanthocyanidin-inhibited coal was reduced by 59.72% and that in polyethylene glycol-inhibited coal was reduced by 24.37%, demonstrating the stronger effect of the proanthocyanidin. The hydrogen bond peaks are found in the 3624–3610 cm−1 range. It is observed from the results presented in Table 4 that there were no hydrogen bond associated with the hydroxyl groups in the raw coal samples, where most of the hydroxyl groups are free hydroxyl groups with higher activity. The hydrogen bond peak of the hydroxyl-associated synthesis appears in the coal samples treated with proanthocyanidin and polyethylene glycol, that is hydroxyl-associated hydrogen bonds which is observed with the peak areas of 0.325 and 0.205, respectively, indicating that the structure in the proanthocyanidins- and polyethylene-glycols-treated samples forms hydrogen bonds with the hydroxyl groups in the coal, reducing the amount of free hydroxyl groups in the coal sample and decreasing the activity of the hydroxyl groups. Therefore, OPC and polyethylene glycols have a significant inhibitory effect on the hydroxyl activity and effectively reduce the oxidative self-ignitability of coal.
(3) Line width H The process of the recovery of the electron distribution from the unbalanced state to the equilibrium state is known as the relaxation process, and the time required to reach the equilibrium state is known as the relaxation time. The line width is inversely proportional to the relaxation time, and the relaxation time is directly proportional to the concentration. The line widths of the different groups of coal samples are in the order of raw coal sample < polyethylene glycol inhibitory coal sample < proanthocyanidin inhibitory coal sample. It is inferred that this order may be observed because the relaxation time decreases with the decreasing free radicals concentration, resulting in the broadening of the spectral line; alternatively, it is possible that due to the weakening of electron exchange between the free radicals, the relaxation time is shortened, and the spectral line is broadened. 3.1.3. Fourier infrared spectroscopy investigations of the coal sample Fig. 10 show the infrared spectra of the raw coal and the coal samples with different inhibitors. According to the attribution of infrared absorption peak of coal [33–35], the experimental results were analyzed qualitatively and quantitatively. (1) Qualitative analysis of the infrared spectra 5
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Table 2 Peak area of main characteristic peaks of infrared spectrum of coal sample. Number
1 2 3 4 5 6 7 8
Functional group
Coal sample
Free and associated hydroxyl OH Hydroxyl-linked hydrogen bond Fat base, eCH2, eCH3 Phenol, alcohol, ether, ester oxygen bond Ar-C-O C-O-C in ethers Carboxyl group, eCOOH Aromatic hydrocarbon eCH, C]C, CeH Carbonyl, C]O
Raw coal sample
OPC-inhibited coal sample
PEG-inhibited coal sample
1.863 0 2.875 1.914 1.782 1.769 3.934
0.750 0.325 2.766 1.735 2.543 1.688 4.819
1.409 0.205 2.873 1.840 2.110 1.687 5.367
0.326
0.292
0.315
The contents of oxygen-containing functional groups and fatty groups decreased somewhat after the inhibiting treatment with OPC and polyethylene glycol. OPC had the most pronounced effect with the reduction of the phenol, alcohol, ether, ester-oxygen bond and carbonyl content by 9.35% and 10.43%, respectively. Polyethylene glycol had a significant effect with the reduction of the carboxyl content by 4.64%. The aliphatic groups content in the proanthocyanidin-inhibited coal decreased by 3.79%, while that in the polyethylene glycol-inhibited coal decreased slightly. The CeOeC content of ethers in the coal samples increased by 42.71% and 18.41% after the inhibiting treatments with proanthocyanidin and polyethylene glycol, respectively. The increase in the ether bonds in the coal samples inhibited by proanthocyanidin was approximately 2.3 times than that obtained by using polyethylene glycol. After the OPC and polyethylene glycol inhibition treatment, the amount of stable aromatic hydrocarbons in the coal samples also increased to a certain extent. The amount of the aromatic hydrocarbons in the polyethylene-glycol-inhibited coal samples showed a significant increase of 36.43%.
Table 4 Starting point and peak position of hydroxyl-linked hydrogen bond in coal sample. Wave number/cm−1
Beginning Ending Peak position
Coal sample Raw coal
OPC inhibition coal sample
PEG inhibition coal sample
3608 3641 3626
3552 3662 3624
3606 3643 3624
3.2. Experimental study of inhibition by a compound inhibitor 3.2.1. Programmed oxidation and heating test of the coal spontaneous combustion (1) Crossing-point temperature The crossing-point temperatures of the composite chemical inhibitors added with different proportions of 10% were measured using a programmed oxidation and temperature system with the results shown in Fig. 11 (adding single inhibitors and raw coal for contrastive analysis), and the plots of the rise in the coal sample temperature are shown in Figs. 12–15. An examination of the data presented in Fig. 11 shows that the
Fig. 11. Crossing-point temperature of the coal inhibited with a compound inhibitor.
Table 3 Change in the peak area of functional groups of the inhibited coal samples. Number
Functional group
Coal sample OPC-inhibited coal sample
1 2 3 4 5 6 7 8
Free and associated hydroxyl OH Hydroxyl-linked hydrogen bond Fat base, eCH2, eCH3 Phenol, alcohol, ether, ester oxygen bond AreCeO CeOeC in ethers Carboxyl group, eCOOH Aromatic hydrocarbon eCH, C]C, CeH Carbonyl, C]O
PEG-inhibited coal sample
Increase or decrease
Percentage/%
Increase or decrease
Percentage/%
−1.113 0.325 −0.109 −0.179 0.761 −0.081 0.885
59.742
24.369
3.791 9.352 42.705 4.579 22.496
−0.454 0.205 −0.002 −0.074 0.328 −0.082 1.433
0.070 3.866 18.406 4.635 36.426
−0.034
10.429
−0.011
3.374
6
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Fig. 12. Temperature rise plot for the inhibited coal sample treated with the compound ratio of 1:1.
Fig. 15. Temperature rise plot of the inhibited coal sample treated with the compound ratio of 1:8.
play an obvious role in the inhibition. Based on the obtained experimental values of the crossing-point temperature of the composite chemical inhibitors, it was concluded that the crossing-point temperature of the coal samples is increased by 16.9 °C, 25 °C, 19.3 °C and 13.4 °C, respectively. When the ratio of proanthocyanidin to polyethylene glycol is 1:2, the crossing-point temperature is higher than those for the other mixing ratios of the composite chemical inhibitors. And the crossing-point temperature is 170.2 °C, 7.4 °C and 10.7 °C higher than those of the single inhibitors which are at 162.8 °C and 159.5 °C, respectively. According to the temperature rise plot for the coal sample with the compound chemical retardant, the crossing-point temperature of raw coal appears after approximately 120 min. As observed from Fig. 13, the composite chemical inhibitor with the ratio of 1:2 has a crossingpoint temperature at approximately 140 min, while Figs. 12 and 14 show that the composite chemical inhibitors with the ratio of 1:1 and 1:4 have a crossing-point temperature near 130 min, which is later than that of the raw coal. The compound chemical retardant with the ratio of 1:2 shows a good effect for raising the crossing-point temperature of the coal sample and reducing the spontaneous combustion tendency of the coal sample.
Fig. 13. Temperature rise plot for the inhibited coal sample treated with the compound ratio of 1:2.
(2) CO and CO2 gas concentration Fig. 16 shows the curve for the CO gas concentration as a function of temperature obtained by the programmed oxidation and temperature measurements of the coal sample treated with composite chemical inhibitor. Carbon monoxide (CO) production in the oxidation process of the coal samples with compound chemical inhibitors increased with the increase of the oxidation temperature. At 100 °C, the amount of carbon monoxide produced by the coal treated with the composite chemical inhibitor with the compound ratio of 1:2 was 1814 ppm lower than that of the raw coal, and 971.8 ppm and 1384.9 ppm lower than those of the coal samples treated with OPC and polyethylene glycol single inhibitors, respectively and was 47.6 ppm, 310.2 ppm and 397.2 ppm lower than those of the coal samples treated with the compound ratios of 1:1, 1:4 and 1:8, respectively. The carbon monoxide concentration curve of the coal sample treated with compound chemical inhibitor with the compound ratio of 1:2 tends to be gentle after 100 °C, and carbon monoxide production is always lower than those for the samples with proanthocyanidin or polyethylene glycol single inhibitors and other complex chemical inhibitors. Fig. 17 shows the temperature dependence of the CO2 concentration
Fig. 14. Temperature rise plot for the inhibited coal sample treated with the compound ratio of 1:4.
crossing-point temperature of the inhibited coal sample treated with OPC and PEG is 158.6 °C–170.2 °C, which is obviously higher than that of the raw coal (145.2 °C), indicating that the OPC and PEG inhibitors
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Fig. 16. CO concentrations in the coal samples inhibited by the composite chemical inhibitors.
Fig. 18. Change curve of oxygen consumption rate of coal sample inhibited by composite chemical inhibitor.
consumption rate on the temperature obtained from the programmed temperature and oxidation experiment for the composite chemical retardant inhibiting the coal samples. As the oxidation reaction proceeds, oxygen concentration decreases and the oxygen consumption rate increases. Compared to the original coal, the oxygen consumption rate decreases sharply with the addition of compound chemical inhibitors. At 60 °C, various types of inhibitors have a significant inhibitory effect on the coal samples. At 70 °C, the oxygen consumption rate of the coals treated with the composite chemical inhibitor with the compound ratio of 1:2 is 25.71 × 1011/mol/ (cm3·s) lower than that of the raw coal, is 0.38 × 1011/mol/(cm3·s) and 6.16 × 1011/mol/(cm3·s) lower than those of the coal samples treated with OPC and polyethylene glycol single inhibitors, respectively and is 5.77 × 1011/mol/(cm3·s), 2.69 × 1011/mol/(cm3·s) and 5.00 × 1011/ mol/(cm3·s) lower than those of the coal samples treated with the compound ratios of 1:1, 1:4 and 1:8, respectively. After 70 °C, the oxygen consumption rate change curve of the compound inhibitor with the compound ratio of 1:2 is always lower than those of the OPC and polyethylene glycol single inhibitors and the other three groups of compound inhibitors.
Fig. 17. Change curve of CO2 concentration in coal samples inhibited by composite chemical inhibitor.
obtained from the programmed temperature and oxidation experiment for the coal sample treated with the composite chemical inhibitors. The amount of the released carbon dioxide increased with increasing temperature. The curves for the carbon dioxide concentration of the coal samples treated with the compound chemical inhibitors during the oxidation are lower than that of the raw coal sample. At 110 °C, the amount of carbon dioxide produced by the coal sample treated with the compound chemical inhibitor with the compound ratio of 1:2 is lower than that of the raw coal by 150,547 ppm and is 38,314 ppm and 12,633 ppm lower than those of the coal samples treated with the OPC and polyethylene glycol single inhibitors, respectively and is 129,572 ppm, 20,737 ppm and 49,289 ppm lower than those of the coal samples treated with the complex inhibitors with the compound ratios of 1:1, 1:4 and 1:8, respectively. After 110 °C, all of the inhibitors studied here inhibit the production of carbon dioxide. However, the carbon dioxide concentration curve of the compound inhibitor with the compound ratio of 1:2 is always lower than those of proanthocyanidin and polyethylene glycol inhibitors and the other three groups of compound inhibitors.
3.2.2. Mechanism analysis of compound inhibition According to the above experimental results, both OPC and PEG could effectively suppress coal spontaneous combustion. However, the starting temperature of coal spontaneous combustion inhibition and the ability of preventing coal spontaneous combustion were different. The inhibiting effect of composite inhibitors was more obvious than that of single ones. According to results and discussion, the combination of the two has the effect of synergistic inhibition for the three reasons as below. (1) PEG is in the state of viscous liquid. When combined with OPC powder, it can isolate oxygen, improve the disadvantage of PEG’s easy oxidation, and enhance the chemical inhibition of OPC. At the same time, PEG also has the physical resistance effect of moisturizing and cooling, which can further improve the synergistic inhibition effect. The physical state of PEG and OPC are shown in Fig. 19. (2) PEG is rich in hydroxyl groups, which can be associated with OPC to synthesize larger hydrogen proton donors and thus improve the antioxidant activity of OPC. At the same time, with a good solubility in water, PEG can be well dissolved with OPC, so it can not only be used as a solvent or phase transfer agent, but also can act as resistance reaction catalyst [36] in the reaction process, further
(3) Oxygen consumption rate Fig. 18 shows the curves for the dependence of the oxygen 8
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Fig. 20. Schematic diagram of inhibition mechanism.
polyethylene glycol and proanthocyanidin acting as chemical inhibitors provide hydrogen for the peroxide free radical reaction in the coal to form hydrogen peroxide that readily decomposes into alcohol and water under heating. Polyethylene glycol and proanthocyanidin contain a large number of hydroxyl groups that can etherify with alcohols to form stable ether bonds. In addition, the structure in polyethylene glycol and proanthocyanidin can also associate with the hydroxyl groups in the coal to form hydrogen bonds, thus reducing the activity of the hydroxyl groups. To summarize, polyethylene glycol and proanthocyanidins decrease the oxidation rate of the hydroxyl groups on the coal surface, inhibit the oxidation activity of the coal, and reduce its tendency for spontaneous combustion. The chemical inhibition mechanism of PEG and OPC is illustrated in Fig. 20.
Fig. 19. PEG and OPC.
improving the synergistic inhibition effect. (3) According to the chain reaction process of free radicals in the process of coal spontaneous combustion, combined with experimental result and discussions, it can be inferred that EPG and OPC, as chemical inhibitors, can react with the intermediate products in the process of spontaneous combustion to generate relatively stable ether bonds. The analysis is as follows: Previous work in the field has shown [37] that during the action of the coal oxygenation, the oxygen molecule first adsorbs chemically on the high-activity groups on the coal surface, and a chemical bond in the oxygen molecule weakens or even breaks off under the action of the intermolecular force to form eOeOe that then reacts with eCH3 and eCH2 in the coal molecular structure to form hydrocarbon free radicals. Molecules then react with these hydrocarbon radicals to produce peroxide radicals that react with hydrogen to produce hydrogenated peroxides that decompose into different radicals under heating. The main reaction process is shown in (1), (2) and (3), where R represents a hydrocarbon radical.
R·+O2 → R− O− O·
(1)
R− O− O·+H·→ R− O− OH
(2)
R− O− OH → R− O·+OH·
(3)
4. Conclusions In this study, lignite was studied, and the properties of proanthocyanidin and polyethylene glycol as chemical inhibitors for preventing coal spontaneous combustion were studied [39]. The inhibitory effects of OPC and polyethylene glycols were analyzed macroscopically, and the inhibition properties of OPC and polyethylene glycols were further analyzed microscopically. The following conclusions were drawn: (1) The programmed temperature and oxidation experiments for the coal samples treated with OPC and polyethylene glycol showed that OPC and polyethylene glycol could effectively increase the crossingpoint temperature of the coal samples, noticeably delay the appearance time of the crossing-point temperature, reduce the production of CO and CO2 and the oxygen consumption rate, and show good inhibition performance. The inhibition effect of proanthocyanidin is better than that of polyethylene glycol. (2) The best inhibition effect was obtained for the mixing ratio of OPC to polyethylene glycol of 1:2. At this time, the crossing-point temperature of the coal sample was 25 °C higher than that of the raw coal, while that of the coal sample treated with the proanthocyanidin was increased by 17.6 °C, and that of the coal sample treated with the polyethylene glycol was increased by 14.3 °C. Similarly, the CO, CO2 gas production and oxygen consumption rate results showed that the best inhibition effect was obtained for the compound ratio of 1:2. At this time, the oxidation reaction rate of coal was effectively inhibited, and good results were obtained for reducing the spontaneous combustion of coal samples. (3) The combination of OPC and PEG has a synergistic inhibitory effect. The hydroxyl group in PEG can combine with OPC to synthesize larger hydrogen proton donors by hydrogen bond, and improve the antioxidant activity. PEG can be well soluble with PEG and catalyze the catalytic reaction. The combination can not only enhance the antioxidant capacity of PEG, but also keep moisture and cool the coal surface. (4) The hydroxyl groups of OPC and PEG can be etherified with the reaction product alcohols in the process of coal spontaneous combustion to form ether bonds with relatively stable chemical properties. In addition, the hydrogen contained in PEG and OPC can also be associated with the hydroxyl in coal to synthesize hydrogen bonds, thus reducing the hydroxyl activity and inerting coal oxidation activity.
Alkyl and oxygen-containing radicals in coal are oxidized to aldehyde groups in the coal oxidation process at an elevated temperature. Some of the aldehyde groups decompose to produce CO. Some of these aldehyde groups then are further oxidized to carboxyl groups. The carboxyl groups are decomposed to produce CO2. The reaction process is shown in (4), (5) and (6).
R− O·+H·→ R− OH
(4)
R− OH + O2 → R′CHO → R′·+CO
(5)
R′CHO + O2 → R′COOH → R″·+ CO2
(6)
During the continuous generation and combination of free radicals, a large amount of heat is released and accumulated, leading to the gradual increase in the coal temperature. When the temperature reaches a certain level, the fatty groups and oxygen-containing functional groups in the coal begin to react with oxygen, triggering coal spontaneous combustion. The molecular formula of OPC is C30H12O6, which contains a large number of phenolic hydroxyl groups in its molecular structure, which are more likely to combine with active radicals and oxides, thereby reducing the active transmitter and blocking the free radical chain reaction [38].The molecular formula of PEG is HOe(CH2eCH2eO)neH, and the end groups are two hydroxyl groups with primary alcohol properties, which are prone to etherify with some structures in the process of coal oxidation to form a stable ether bond structure. Based on the above free radical chain reaction process and the structural characteristics of PEG and OPC, it is inferred that 9
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Acknowledgements
for blocking air leakage in coal mine. Russ J Appl Chem 2014;87:1099–108. [18] Hu XM, Wang DM, Wang SL. Synergistic effects of expandable graphite and dimethyl methyl phosphonate on the mechanical properties, fire behavior, and thermal stability of a polyisocyanurate-polyurethane foam. Int J Min Sci Technol 2013;23:13–20. [19] Wu JM, Yan H, Wang JF, Wu YG. Flame retardant polyurethane elastomer nanocomposite applied to coal mines as air-leak sealant. J Appl Polym Sci 2013;129:3390–5. [20] Lu Y, Shi SL, Wang HQ, Tian ZJ, Ye Q, Niu HY. Thermal characteristics of cement microparticle-stabilized aqueous foam for sealing high-temperature mining fractures. Int J Heat Mass Transf 2019;131:594–603. [21] Lu Y. Laboratory study on the rising temperature of spontaneous combustion in coal stockpiles and a paste foam suppression technique. Energy Fuels 2017;31(7):7290–8. [22] Li JL, Lu W, Xu J. Coal spontaneous combustion prevention and cure with chemical retarder as well as analysis on retarding mechanism. Coal Sci Technol 2012;40:50–3. [23] Yang YL, Yu SJ, Zhang RY, Yang H, Fan XL. Study on new stopping agent of preventing coal spontaneous combustion. J China Coal Soc 1999;24:163–6. [24] Monagas M, Quintanillalópez JE, Gómezcordovés C, Bartolomé B, Lebrón-Aguilar R. MALDI-TOF MS analysis of plant proanthocyanidins. J Pharm Biomed Anal 2010;51:358–72. [25] Fracassetti D, Costa C, Moulay L, Tomás-Barberána FA. Ellagic acid derivatives, ellagitannins, proanthocyanidins and other phenolics, vitamin C and antioxidant capacity of two powder products from camu-camu fruit (Myrciaria dubia). Food Chem 2013;139:578–88. [26] Pasut G, Veronese FM. PEG conjugates in clinical development or use as anticancer agents: an overview. Adv Drug Deliv Rev 2009;61:1177–88. [27] Harris JM, Chess RB. Effect of pegylation on pharmaceuticals. Nat Rev Drug Discovery 2003;2:214–21. [28] Greenwald RB. PEG drugs: an overview. J Control Release 2001;74:159–71. [29] Zhang YT, Liu YR, Shi XQ, Yang CP, Wang WF, Li YQ. Risk evaluation of coal spontaneous combustion on the basis of auto-ignition temperature. Fuel 2018;233:68–76. [30] Zhang YT, Li YQ, Huang Y, Li SS, Wang WF. Characteristics of mass, heat and gaseous products during coal spontaneous combustion using TG/DSC–FTIR technology. J Therm Anal Calorim 2018;131:2963–74. [31] Zhang L, Liu WL, Men DP. Preparation and coking properties of coal maceral concentrates. Int J Min Sci Technol 2014;24:93–8. [32] Dou GL. Research on chemical inhibitors for preventing coal spontaneous combustion. China University of Mining and Technology Press; 2014. [33] Feng J, Li WY, Xie KC. Study on coal structure by Fourier infrared spectroscopy. J China Univ Min Technol 2002;31:362–6. [34] Yu ZB, Jiang GL. Analytical chemistry experiments. Beijing: Chemical Industry Press; 2006. [35] Zhang GS, Xie YM, Gu JM. Infrared spectroscopy analysis of coal microstructure changes. J Chin Acad Coal 2003;28:473–6. [36] Liang J. Study on the organic reactions using PEG as green medium. Hangzhou: Zhejiang University; 2012. [37] Deng J, Zhao JY, Huang AC, Zhang YN, Wang CP, Shu CM. Thermal behavior and microcharacterization analysis of second-oxidized coal. J Therm Anal Calorim 2016;127:1–10. [38] Zhang Y, Wu XX. Advances in procyanidins research. Pharm Clin Traditional Chin Med 2011;27:112–6. [39] Zhang YT, Shi XQ, Li YQ, Liu YR. Characteristics of carbon monoxide production and oxidation kinetics during the decaying process of coal spontaneous combustion. Can J Chem Eng 2018;96(8):1752–61.
The authors appreciate the financial support of project No. 51474017 provided by the National Natural Science Foundation of China and project No. E21724 provided by the Work Safety Key Lab on Prevention and Control of Gas and Roof Disasters for Southern Coal Mines of China, as well as project No. WS2018B03 provided by the State Key Laboratory Cultivation Base for Gas Geology and Gas Control (Henan Polytechnic University). References [1] Liu L, Zhou FB. A comprehensive hazard evaluation system for spontaneous combustion of coal in underground mining. Int J Coal Geol 2010;82:27–36. [2] Niu HY, Deng XL, Li SL, Cai KX, Zhu H, Li F, et al. Experiment study of optimization on prediction index gases of coal spontaneous combustion. J Central South Univ 2016;23(9):2321–8. [3] Xia TQ, Zhou FB, Wang XX, Zhang YF, Li YM, Kang JH, et al. Controlling factors of symbiotic disaster between coal gas and spontaneous combustion in longwall mining gobs. Fuel 2016;182:886–96. [4] Pone JDN, Hein KAA, Stracher GB, Annegarn HJ, Finkleman RB, Blake DR, et al. The spontaneous combustion of coal and its by-products in the Witbank and Sasolburg coalfields of South Africa. Int J Coal Geol 2007;72:124–40. [5] Niu HY, Qiao CL, An JY, Deng J. Experimental study and numerical simulation of spread law for fire on tunnel. J Central South Univ 2015;22(2):701–6. [6] Wang J, Anthony EJ. CO oxidation and the inhibition effects of halogen species in fluidised bed combustion. Combust Theor Model 2009;13:105–19. [7] Liodakis S, Bakirtzis D, Lois E, Gakis D. The effect of (NHX4)2HPO4 and (NH4)2SO4 on the spontaneous ignition properties of Pinus halepensis pine needles. Fire Saf J 2002;37:48l–94. [8] Wang DM, Dou GL, Zhong XX, Xin HH, Qin BT. An experimental approach to selecting chemical inhibitors to retard the spontaneous combustion of coal. Fuel 2014;117:218–23. [9] Xu YL, Wang DM, Wang LY, Zhong XX, Chu TX. Experimental research on inhibition performances of the sand-suspended colloid for coal spontaneous combustion. Saf Sci 2012;50:822–7. [10] Xu YL, Wang LY, Chu TX, Liang DL. Suspension mechanism and application of sandsuspended slurry for coalmine fire prevention. Int J Min Sci Technol 2014;24:649–56. [11] Ren WX, Guo Q, Zuo BZ, Wang ZF. Design and application of device to add powdered gelling agent to pipeline system for fire prevention in coal mines. J Loss Prev Process Ind 2016;41:147–53. [12] Deng J, Xiao Y, Lu JH, Wen H, Jin YF. Application of composite fly ash gel to extinguish outcrop coal fires in China. Nat Hazards 2015;79:881–98. [13] Qin BT, Dou GL, Wang Y, Xin HH, Ma LY, Wang DM. A superabsorbent hydrogel–ascorbic acid composite inhibitor for the suppression of coal oxidation. Fuel 2017;190:129–35. [14] Zhou FB, Shi BB, Liu YK, Song XL, Cheng JW, Hu SY. Coating material of air sealing in coal mine: clay composite slurry (CCS). Appl Clay Sci 2013;80–81:299–304. [15] Hu XM, Wang DM. Enhanced fire behavior of rigid polyurethane foam by intumescent flame retardants. J Appl Polym Sci 2013;129:238–46. [16] Hu XM, Zhao YY, Cheng WM, Wang DM. Synthesis and characterization of phenolurea-formaldehyde foaming resin used to block air leakage in mining. Polym Compos 2014;35:2056–66. [17] Hu XM, Cheng WM, Wang DM. Properties and applications of novel composite foam
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Full Length Article
Experimental study on the compound system of proanthocyanidin and polyethylene glycol to prevent coal spontaneous combustion
T
⁎
Zhian Huanga,b,c, Xiaohan Liua, Yukun Gaoa, , Yinghua Zhanga, Ziyou Lia, Hui Wanga, Xinrui Shia a
State Key Laboratory of High-Efficient Mining and Safety of Metal Mines (University of Science and Technology Beijing), Ministry of Education, Beijing 100083, China Work Safety Key Lab on Prevention and Control of Gas and Roof Disasters for Southern Coal Mines (Hunan University of Science and Technology), Xiangtan 411201, China c State Key Laboratory Cultivation Base for Gas Geology and Gas Control (Henan Polytechnic University), Jiaozuo 454000, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Coal spontaneous combustion Programmed temperature Chemical inhibitor Electron spin resonance Infrared spectrum
In order to overcome the short inhibition duration of physical inhibitor, the environmentally friendly chemical inhibitors of polyethylene glycol (PEG) and proanthocyanidin (OPC) were selected to conduct the experimental study for coal spontaneous combustion prevention. Through temperature programmed experiment, electron spin resonance experiment and Fourier infrared spectroscopy experiment, it was found that for the coal sample after treated by PEG and OPC, crossing point temperature increased by 17.6 °C and 14.3 °C, free radical concentration decreased to 0.1512 Ng/1017 g−1 and 0.1178 Ng/1017 g−1, association and hydroxyl free hydroxyl groups decreased by 59.72% and 24.37%, and the stability of the CeOeC ether bond increased by 42.71% and 18.41%, respectively, which showed the inhibition effects of both inhibitors are obvious and OPC is better. For the sample treated by OPC and PEG (ratio 1:2), crossing point temperature increased 25 °C, which indicated that the collaborative effect was remarkable. The analysis shows that PEG and OPC contain a large number of hydroxyl groups, which react with the oxide intermediate product alcohol to form ether bonds with relatively stable chemical properties, thus mitigating the rate of coal spontaneous combustion; PEG and OPC are linked by hydrogen bonds to synthesize larger hydrogen proton donors, which improved antioxidant activity. PEG is a viscous liquid, when carrying proanthocyanidin, can cover the surface of coal, isolating coal surface from oxygen, keeping moisture and cooling. The combination of the two inhibitors has significant synergistic inhibition effect, which can greatly suppress the coal spontaneous combustion.
1. Introduction Coal industry plays an important role in economic development. Unfortunately, fires caused by spontaneous combustion are a worldwide problem for the coal industry [1–5]. For the suppression of spontaneous combustion of coal, the inhibitor fire prevention technology has been widely applied, obtaining good results. Based on recent scientific research results, the main inhibitors for coal spontaneous combustion are the following: (1) halogen salt and ammonium salt inhibitor [6–8]; (2) gel inhibitor [9–11]; (3) compound inhibitor [12–14]; (4) polymer inhibitor [15–19]; and (5) foam inhibitor [20,21]. The inhibiting mechanism of this kind of inhibitors is mainly physical inhibition, which has disadvantages such as low inhibition efficiency and short inhibition life. However, chemical inhibitors prevent spontaneous combustion of coal by destroying or reducing the structure with higher oxidation reactivity or eliminating free radicals in coal in
⁎
advance. Compared with physical inhibitors, chemical inhibitors have a longer inhibition period [22]. The research for the chemical inhibitors is currently at the initial stage, mainly some antioxidants [23] and metal compounds [22]. The former can control oxide free radical chain reaction, such as naphthylamine, naphthol, diphenylamine three phenol, hydroquinone and PCB, and the latter includes perchlorate, potassium permanganate, sodium persulfate, etc. Research results are relatively less, especially the environmental performance, inhibition characteristics and inhibition mechanism needs to be improved, and further research. In this study, the environment-friendly proanthocyanidin (OPC) and polyethylene glycol (PEG) were selected to investigate inhibiting properties and inhibiting mechanism. Proanthocyanidins (OPC), also known as condensed tannins, are polyphenolic compound, widely contained in the nucleus, skin or seeds of plants and they are among the most abundant polyphenols in our diet. Besides their participation in food quality attributes such as
Corresponding author. E-mail address: [email protected] (Y. Gao).
https://doi.org/10.1016/j.fuel.2019.06.018 Received 3 February 2019; Received in revised form 2 May 2019; Accepted 5 June 2019 Available online 18 June 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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astringency, bitterness, aroma and color formation, proanthocyanidin consumption has been associated with numerous health benefits due to their antioxidant, anti-carcinogenic, cardioprotective, antimicrobial and neuro-protective activities. As a result, they are considered as functional ingredients in botanical and nutritional supplements [24]. And it comes from a wide variety of source and is easy to obtain. In addition, proanthocyanidins are powdery at room temperature and easy to transport. At the same time, OPCs have strong antioxidant and free radical scavenging activities. Many phenolic hydroxyl groups in proanthocyanidins release H+ to competitively combine with free radicals and oxides, so as to block free radical chain reactions. Experiments have shown that the scavenging activity of proanthocyanidins and their metabolites was generally stronger than that of Vitamin C and Vitamin E [25]. Polyethylene glycol (PEG) is a linear polyether composed of several ethoxy repeating units. PEG has a narrow molecular weight distribution and can be dissolved in aqueous solutions and other organic solvents. Due to its unique physical and chemical properties, polyethylene glycol has been widely used [26–28]. At room temperature, PEG-200 is a colorless, odorless and viscous liquid, soluble in water, ethanol and many other organic solvents. PEG-200 has good thermal stability, lubricity, moisture retention and dispersion. It is not easy to hydrolyze, and nontoxic and nonirritating. In addition, PEG-200 is widely used in industry as moisturizer, softener, antistatic agent, etc. The hydroxyl groups in the polyethylene glycol structure can etherify with the hydroxyl groups on the coal surface, and the ether bond is relatively stable in the coal oxidation process. Therefore, polyethylene glycol can remove the active hydroxyl functional groups on the coal surface. Moreover, PEG is stable in highly oxidizing conditions, such as in acidic, alkali, high temperature, and oxygen and hydrogen peroxide rich environments, and has low flammability and good biodegradability. Based on the theory of free radical chain reaction of coal spontaneous combustion, lignite was used as the subject of this study, and proanthocyanidin and polyethylene glycol were selected as the environmentally friendly inhibitors to carry out the experimental investigation of the chemical inhibitor mechanism of coal spontaneous combustion. The temperature programmed system was used to simulate the conditions and environment of low-temperature oxidation spontaneous combustion of coal [29,30], and the index gases produced by coal oxidation were analyzed by gas chromatography. The changes of the free radicals in the coal before and after the inhibition were analyzed by electron spin resonance (ESR) spectroscopy, and the changes of the number of active functional groups in the molecular structure of the coal after inhibition were qualitatively and quantitatively analyzed by infrared spectroscopy. In this study, the single inhibitory effect and the compound inhibitory effect of proanthocyanidin and polyethylene glycol were studied, the optimal compound was determined, and the inhibitory mechanism was analyzed.
Fig. 1. Three prepared coal samples.
absorption of polyethylene to make it dry naturally at room temperature for 48 h. According to the literature [32], when the amount of OPC and PEG200 is 10%, the inhibiting effect is the best. Therefore, coal (1 kg) was weighed into a beaker and 10% proanthocyanidins by weight (OPC, 100 g) was added, mixed well and let stand for 24 h to obtain the coal sample. Coal (1 kg) was weighed into a beaker and 10% polyethylene glycol by weight (PEG-200, 100 g) was added, mixed well and let stand for 24 h to obtain another coal sample. The three prepared coal samples are shown in Fig. 1. 2.2. Experimental study of inhibition by a single inhibitor 2.2.1. Programmed temperature and oxidation experiment of coal spontaneous combustion Low-temperature oxidation of coal is a necessary stage of coal spontaneous combustion, and the study of this stage can explore the direct cause of coal spontaneous combustion. In this study, XCT-0 temperature programmed system was used to simulate the conditions and environment of low-temperature oxidation spontaneous combustion of coal, and the temperature programmed experiment was conducted on raw coal and coal samples treated by inhibitors to study the inhibiting effect. In this experiment, the dosage was 1 kg for each coal sample. Air bottle was used to supply air, and the control gas flow was 100 ml/min, the gas pressure was 0.1 MPa. The temperature rise ranged from 30 °C to 130 °C, the heating rate was 0.5 °C /min. The end of the air outlet tube was connected with a high temperature resistant hose, and this hose was connected with a 500 ml gas sampling bag, and the generated gas was sampled every 10 °C for a total of 11 times. Then the gas was extracted with a medical syringe and injected into an Agilent 7820A high-precision gas chromatographic analyzer to analyze the gas composition and concentration. The central temperature of the coal sample was monitored by thermocouple to record the temperature changes during the experiment.
2. Experiment
2.2.2. Experimental study of free radical of the coal samples by ESR A mixture of dried coal samples with the diameter of 0.2 mm or less was selected, and the dosage of 2 mg was used for each experiment. A JES-FA200 EER Spectrometer (Japan) electron spin resonance spectrometer with the microwave frequency of 9041.47 MHz, microwave power of 1 mW, central center magnetic field of 323.144 mT, scanning width of 5 mT, modulation frequency of 100 kHz, modulation width of 0.01 mT, time constant of 0.03 s, scanning time of 1 min and magnification of 100 times was used. The experiments were conducted at room temperature.
2.1. Preparation of experimental materials The preparation procedures of the coal samples included crushing, screening, mixing, shrinkage and air drying carried out according to GB474-2008 “Preparation method of Coal Samples”, the safety production industry standard of the People's Republic of China [31]. Brown coal with the typical ignition point of 267–300 °C in the Hailar Baozhileer Mine, Inner Mongolia was used for the experiment. A jaw crusher was used to crush the coal samples. The coal samples with the particle sizes of 0–0.9 mm, 0.9–3 mm, 3–5 mm, 5–7 mm and 7–10 mm were sifted using a vibrating screen. The coal samples with different particle sizes were weighed to obtain 0.2 kg of coal for each sample. After fully mixing and separating, it was put into the sealed bag made of polyethylene to isolate oxygen and make use of the water
2.2.3. Fourier infrared spectroscopy experiment of the coal samples A mixed coal sample with the particle size of 0.2 mm or less were selected, and completely cover the ruby in the center of the platform for 2
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each experiment. A VERTEX 70 Fourier infrared spectrometer was used. The scanning wavenumber range was 600–4000 cm−1, the resolution was 4 cm−1, the scanning time of the sample was 2 min, and the background scanning time was 2 min. The experiments were conducted at room temperature. 2.3. Experiment investigation of the compound inhibitor OPC are easy to oxidize, and PEG is in the state of viscous liquid naturally. When the two are combined, PEG can play the role of covering and wrapping the coal surface, isolating oxygen, and can play a better role with OPC. A raw coal sample (1 kg) was placed into a beaker and 10% by weight of the compound inhibitor was added. The proportions of proanthocyanidins to polyethylene glycol were 1:1, 1:2, 1:4 and 1:8. The composite inhibitor was mixed with the raw coal sample uniformly and completely, placed into a sealed bag and stored for 24 h. The programmed temperature and oxidation system used to study the compound inhibitor was the same as that used for the inhibited coal sample as described above.
Fig. 3. Variation of the furnace temperature and center of the coal temperature with time for the OPC-inhibited coal sample.
3. Results and discussion 3.1. Experimental study of inhibition by a single inhibitor 3.1.1. Programmed oxidation and heating test of coal spontaneous combustion (1) crossing-point temperature It can be observed from the results of the programmed oxidation and heating test of raw coal that as the temperature increases, the crossing-point temperature of the raw coal sample appears at approximately 120 min (Fig. 2); in other words, the crossing-point temperature of the raw coal is 145.2 °C. After the addition of OPC as a retarding agent, the cross-point temperature of the raw coal appears at approximately 130 min (Fig. 3), effectively delaying the time of the appearance of the crossing-point temperature of the coal sample. The crossing-point temperature of the coal sample treated with inhibitors is 162.8 °C, which is 17.6 °C higher than that of the raw coal. It can be inferred that OPC plays a certain role in inhibiting the oxidation reaction of lignite and reducing the heat accumulation process. From the programmed temperature experiment on the raw coal, it was observed that the temperature of the center of the raw coal sample and the furnace temperature reached the maximum temperature
Fig. 4. Variation of the furnace temperature and center of the coal temperature with time for the PEG-inhibited coal sample.
difference at approximately 80 min. When polyethylene glycol was added as the inhibitor (Fig. 4), the temperature difference between the central temperature and the furnace temperature of the coal sample reaches the maximum at 100 °C, which is greater than the corresponding temperature difference of the raw coal sample. The crossingpoint temperature of the coal sample treated with inhibitors is 159.5 °C, which is higher than that of the raw coal sample by 14.3 °C (Fig. 5), indicating that polyethylene glycol plays the role of an inhibitor in the oxidation process of lignite and reduces the tendency for coal spontaneous combustion. (2) CO and CO2 gas concentration analysis Figs. 6 and 7 show the curves of the CO and CO2 concentrations with respect to the temperature obtained by programmed oxidation and temperature experiment of proanthocyanidin-inhibited coal samples and polyethylene glycol-inhibited coal samples. Fig. 6 shows that carbon monoxide concentration increases with increasing temperature. The carbon monoxide concentration curve of the coal treated with OPC and polyethylene glycol is always below that of the raw coal. Starting at 100 °C, the difference between the curves of the proanthocyanidin-inhibited coal and raw coal gradually increases, and the inhibition effect of the OPC on the oxidation reaction of the coal is more obvious. A distinct difference is observed between the curves for the polyethylene
Fig. 2. Furnace temperature and center of the coal temperature as the functions of time in the programmed temperature test of raw coal. 3
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Fig. 5. The intersecting temperature of three coal samples.
Fig. 8. Oxygen consumption rates of the OPC- and PEG-inhibited coal samples with temperature.
during the oxidation process. The carbon dioxide concentration curves of the coals treated with OPC and polyethylene glycol are always below the carbon dioxide concentration curves of the raw coal samples. When the temperature rises to 100 °C, the difference between the curves of OPC and raw coal increases gradually, and the difference between the curves of polyethylene glycol and raw coal clearly increases. It was concluded that the inhibition effect of polyethylene glycol is good, but the inhibition effect of OPC is more obvious. (3) Oxygen consumption rate The oxygen consumption rates of proanthocyanidins- and polyethylene-glycol-inhibited coal samples for different temperatures were obtained by the programmed temperature test (Fig. 8). It is observed from Fig. 8 that the oxygen consumption rate of the coal samples clearly decreases after the OPC and polyethylene glycol treatment at the same temperature, and the curve for the oxygen consumption rate as a function of temperature is relatively gentle, reducing the oxygen consumption rate of the coal sample and inhibiting the oxidation reaction.
Fig. 6. CO concentrations of OPC- and PEG-inhibited coal samples plotted versus the temperature.
3.1.2. Experimental study of free radicals of coal samples by ESR Fig. 9 shows the ESR spectra of the raw coal sample and coal samples treated by OPC and polyethylene glycol obtained in electron spin resonance experiments. The abscissa is the intensity of the magnetic field, and the ordinate is the strength of the primary differential curve of the absorption line. The main parameters of the ESR spectrum
Fig. 7. CO2 concentration of OPC- and PEG-inhibited coal samples plotted versus temperature.
glycol-inhibited coal and raw coal, and the amount of carbon monoxide reduction is clearly observed. Thus, it was found that polyethylene glycol inhibits the oxidation reaction of coal to some extent. As shown in Fig. 7, the concentration of carbon dioxide increases
Fig. 9. ESR spectra of the coal samples. 4
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Table 1 Main parameters of the ESR spectra of the coal samples. Coal sample
Free radical concentration Ng/1017 g−1
Radical concentration increment Ng/1017 g−1
g value
Line width △H/mT
Raw coal Add PEG Add OPC
7.8332 7.7154 7.6820
– −0.1178 −0.1512
2.00263 2.00261 2.00259
596.2 596.8 598.3
are shown in Table 1. (1) Free radical concentration Ng According to the free radical chain reaction theory, free radicals are produced and consumed continuously in the interaction of the coal and oxygen. If the rate of free radical generation is higher than the rate of consumption, the free radical concentration will increase, otherwise, it will decrease. The obtained experimental data show that the total concentration of free radicals in the coal decreases with the addition of inhibitors. It is inferred that PEG and OPG inhibitors can effectively remove active free radicals in the coal, and accelerate the termination rate of the free radical chain reaction, thus suppressing the oxidation activity of the coal, inhibiting the free radical reaction between the coal and oxygen, and achieving the goal of preventing coal spontaneous combustion. Based on the obtained values for the reduction in the free radical concentration, the inhibition effect of OPC on the free radical reaction is stronger than that of polyethylene glycol.
Fig. 10. Infrared spectrum of three coal samples.
An examination of the infrared spectroscopy data shows that the infrared spectroscopy absorption peaks of the coal samples are mainly distributed in the vibration frequency region of aromatic hydrocarbons; the high content of aromatic hydrocarbons ensures the stability of the molecular structure of the coal. The infrared absorption peaks that affect the oxidation performance of the coal samples are mainly distributed in the vibration frequency range of fat-based and oxygencontaining functional groups. After treatment with the inhibitor, the absorption peak quantities decreased in the infrared absorption region that affects the oxidation performance of the coal samples.
(2) g factor value
(2) Quantitative analysis of the infrared spectra
The g factor represents the number of free radical species in coal samples. A larger g value indicates that more different free radicals are present in the coal samples. It is observed from the molecular structure of coal that the unpaired electrons in the coal are more localized at the C, H, O, N, P, and S atoms and that the coal may also contain metal complex radicals. An examination of the data presented in Table 1 shows that the obtained g factor values are in the order of the raw coal sample > the polyethylene glycol resistive coal sample > the proanthocyanidin gas resistive coal sample. It is inferred that the active radicals in the inhibitor are combined with the active radicals in the raw coal, reducing the total content of different radicals.
The infrared spectra were quantitatively analyzed by setting the spectral baseline and calculating the area surrounded by the characteristic absorption peak and the baseline to determine the amount of the corresponding functional groups. To further determine the changes in the functional group structure of the coal sample treated with inhibitors and the original coal sample, the characteristic peak areas of the infrared spectra of the coal samples were calculated using the ORIGIN software. The results are shown in Table 2. The functional group changes of the inhibited coal sample relative to the original coal sample are shown in Table 3. An examination of the data presented in Tables 2 and 3 shows that many free hydroxyl and associative hydroxyl groups are present in raw coal, whereas the content of free hydroxyl and associative hydroxyl groups in the coal treated with the inhibitor is clearly reduced. The content of the free hydroxyl and associative hydroxyl in the proanthocyanidin-inhibited coal was reduced by 59.72% and that in polyethylene glycol-inhibited coal was reduced by 24.37%, demonstrating the stronger effect of the proanthocyanidin. The hydrogen bond peaks are found in the 3624–3610 cm−1 range. It is observed from the results presented in Table 4 that there were no hydrogen bond associated with the hydroxyl groups in the raw coal samples, where most of the hydroxyl groups are free hydroxyl groups with higher activity. The hydrogen bond peak of the hydroxyl-associated synthesis appears in the coal samples treated with proanthocyanidin and polyethylene glycol, that is hydroxyl-associated hydrogen bonds which is observed with the peak areas of 0.325 and 0.205, respectively, indicating that the structure in the proanthocyanidins- and polyethylene-glycols-treated samples forms hydrogen bonds with the hydroxyl groups in the coal, reducing the amount of free hydroxyl groups in the coal sample and decreasing the activity of the hydroxyl groups. Therefore, OPC and polyethylene glycols have a significant inhibitory effect on the hydroxyl activity and effectively reduce the oxidative self-ignitability of coal.
(3) Line width H The process of the recovery of the electron distribution from the unbalanced state to the equilibrium state is known as the relaxation process, and the time required to reach the equilibrium state is known as the relaxation time. The line width is inversely proportional to the relaxation time, and the relaxation time is directly proportional to the concentration. The line widths of the different groups of coal samples are in the order of raw coal sample < polyethylene glycol inhibitory coal sample < proanthocyanidin inhibitory coal sample. It is inferred that this order may be observed because the relaxation time decreases with the decreasing free radicals concentration, resulting in the broadening of the spectral line; alternatively, it is possible that due to the weakening of electron exchange between the free radicals, the relaxation time is shortened, and the spectral line is broadened. 3.1.3. Fourier infrared spectroscopy investigations of the coal sample Fig. 10 show the infrared spectra of the raw coal and the coal samples with different inhibitors. According to the attribution of infrared absorption peak of coal [33–35], the experimental results were analyzed qualitatively and quantitatively. (1) Qualitative analysis of the infrared spectra 5
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Table 2 Peak area of main characteristic peaks of infrared spectrum of coal sample. Number
1 2 3 4 5 6 7 8
Functional group
Coal sample
Free and associated hydroxyl OH Hydroxyl-linked hydrogen bond Fat base, eCH2, eCH3 Phenol, alcohol, ether, ester oxygen bond Ar-C-O C-O-C in ethers Carboxyl group, eCOOH Aromatic hydrocarbon eCH, C]C, CeH Carbonyl, C]O
Raw coal sample
OPC-inhibited coal sample
PEG-inhibited coal sample
1.863 0 2.875 1.914 1.782 1.769 3.934
0.750 0.325 2.766 1.735 2.543 1.688 4.819
1.409 0.205 2.873 1.840 2.110 1.687 5.367
0.326
0.292
0.315
The contents of oxygen-containing functional groups and fatty groups decreased somewhat after the inhibiting treatment with OPC and polyethylene glycol. OPC had the most pronounced effect with the reduction of the phenol, alcohol, ether, ester-oxygen bond and carbonyl content by 9.35% and 10.43%, respectively. Polyethylene glycol had a significant effect with the reduction of the carboxyl content by 4.64%. The aliphatic groups content in the proanthocyanidin-inhibited coal decreased by 3.79%, while that in the polyethylene glycol-inhibited coal decreased slightly. The CeOeC content of ethers in the coal samples increased by 42.71% and 18.41% after the inhibiting treatments with proanthocyanidin and polyethylene glycol, respectively. The increase in the ether bonds in the coal samples inhibited by proanthocyanidin was approximately 2.3 times than that obtained by using polyethylene glycol. After the OPC and polyethylene glycol inhibition treatment, the amount of stable aromatic hydrocarbons in the coal samples also increased to a certain extent. The amount of the aromatic hydrocarbons in the polyethylene-glycol-inhibited coal samples showed a significant increase of 36.43%.
Table 4 Starting point and peak position of hydroxyl-linked hydrogen bond in coal sample. Wave number/cm−1
Beginning Ending Peak position
Coal sample Raw coal
OPC inhibition coal sample
PEG inhibition coal sample
3608 3641 3626
3552 3662 3624
3606 3643 3624
3.2. Experimental study of inhibition by a compound inhibitor 3.2.1. Programmed oxidation and heating test of the coal spontaneous combustion (1) Crossing-point temperature The crossing-point temperatures of the composite chemical inhibitors added with different proportions of 10% were measured using a programmed oxidation and temperature system with the results shown in Fig. 11 (adding single inhibitors and raw coal for contrastive analysis), and the plots of the rise in the coal sample temperature are shown in Figs. 12–15. An examination of the data presented in Fig. 11 shows that the
Fig. 11. Crossing-point temperature of the coal inhibited with a compound inhibitor.
Table 3 Change in the peak area of functional groups of the inhibited coal samples. Number
Functional group
Coal sample OPC-inhibited coal sample
1 2 3 4 5 6 7 8
Free and associated hydroxyl OH Hydroxyl-linked hydrogen bond Fat base, eCH2, eCH3 Phenol, alcohol, ether, ester oxygen bond AreCeO CeOeC in ethers Carboxyl group, eCOOH Aromatic hydrocarbon eCH, C]C, CeH Carbonyl, C]O
PEG-inhibited coal sample
Increase or decrease
Percentage/%
Increase or decrease
Percentage/%
−1.113 0.325 −0.109 −0.179 0.761 −0.081 0.885
59.742
24.369
3.791 9.352 42.705 4.579 22.496
−0.454 0.205 −0.002 −0.074 0.328 −0.082 1.433
0.070 3.866 18.406 4.635 36.426
−0.034
10.429
−0.011
3.374
6
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Fig. 12. Temperature rise plot for the inhibited coal sample treated with the compound ratio of 1:1.
Fig. 15. Temperature rise plot of the inhibited coal sample treated with the compound ratio of 1:8.
play an obvious role in the inhibition. Based on the obtained experimental values of the crossing-point temperature of the composite chemical inhibitors, it was concluded that the crossing-point temperature of the coal samples is increased by 16.9 °C, 25 °C, 19.3 °C and 13.4 °C, respectively. When the ratio of proanthocyanidin to polyethylene glycol is 1:2, the crossing-point temperature is higher than those for the other mixing ratios of the composite chemical inhibitors. And the crossing-point temperature is 170.2 °C, 7.4 °C and 10.7 °C higher than those of the single inhibitors which are at 162.8 °C and 159.5 °C, respectively. According to the temperature rise plot for the coal sample with the compound chemical retardant, the crossing-point temperature of raw coal appears after approximately 120 min. As observed from Fig. 13, the composite chemical inhibitor with the ratio of 1:2 has a crossingpoint temperature at approximately 140 min, while Figs. 12 and 14 show that the composite chemical inhibitors with the ratio of 1:1 and 1:4 have a crossing-point temperature near 130 min, which is later than that of the raw coal. The compound chemical retardant with the ratio of 1:2 shows a good effect for raising the crossing-point temperature of the coal sample and reducing the spontaneous combustion tendency of the coal sample.
Fig. 13. Temperature rise plot for the inhibited coal sample treated with the compound ratio of 1:2.
(2) CO and CO2 gas concentration Fig. 16 shows the curve for the CO gas concentration as a function of temperature obtained by the programmed oxidation and temperature measurements of the coal sample treated with composite chemical inhibitor. Carbon monoxide (CO) production in the oxidation process of the coal samples with compound chemical inhibitors increased with the increase of the oxidation temperature. At 100 °C, the amount of carbon monoxide produced by the coal treated with the composite chemical inhibitor with the compound ratio of 1:2 was 1814 ppm lower than that of the raw coal, and 971.8 ppm and 1384.9 ppm lower than those of the coal samples treated with OPC and polyethylene glycol single inhibitors, respectively and was 47.6 ppm, 310.2 ppm and 397.2 ppm lower than those of the coal samples treated with the compound ratios of 1:1, 1:4 and 1:8, respectively. The carbon monoxide concentration curve of the coal sample treated with compound chemical inhibitor with the compound ratio of 1:2 tends to be gentle after 100 °C, and carbon monoxide production is always lower than those for the samples with proanthocyanidin or polyethylene glycol single inhibitors and other complex chemical inhibitors. Fig. 17 shows the temperature dependence of the CO2 concentration
Fig. 14. Temperature rise plot for the inhibited coal sample treated with the compound ratio of 1:4.
crossing-point temperature of the inhibited coal sample treated with OPC and PEG is 158.6 °C–170.2 °C, which is obviously higher than that of the raw coal (145.2 °C), indicating that the OPC and PEG inhibitors
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Fig. 16. CO concentrations in the coal samples inhibited by the composite chemical inhibitors.
Fig. 18. Change curve of oxygen consumption rate of coal sample inhibited by composite chemical inhibitor.
consumption rate on the temperature obtained from the programmed temperature and oxidation experiment for the composite chemical retardant inhibiting the coal samples. As the oxidation reaction proceeds, oxygen concentration decreases and the oxygen consumption rate increases. Compared to the original coal, the oxygen consumption rate decreases sharply with the addition of compound chemical inhibitors. At 60 °C, various types of inhibitors have a significant inhibitory effect on the coal samples. At 70 °C, the oxygen consumption rate of the coals treated with the composite chemical inhibitor with the compound ratio of 1:2 is 25.71 × 1011/mol/ (cm3·s) lower than that of the raw coal, is 0.38 × 1011/mol/(cm3·s) and 6.16 × 1011/mol/(cm3·s) lower than those of the coal samples treated with OPC and polyethylene glycol single inhibitors, respectively and is 5.77 × 1011/mol/(cm3·s), 2.69 × 1011/mol/(cm3·s) and 5.00 × 1011/ mol/(cm3·s) lower than those of the coal samples treated with the compound ratios of 1:1, 1:4 and 1:8, respectively. After 70 °C, the oxygen consumption rate change curve of the compound inhibitor with the compound ratio of 1:2 is always lower than those of the OPC and polyethylene glycol single inhibitors and the other three groups of compound inhibitors.
Fig. 17. Change curve of CO2 concentration in coal samples inhibited by composite chemical inhibitor.
obtained from the programmed temperature and oxidation experiment for the coal sample treated with the composite chemical inhibitors. The amount of the released carbon dioxide increased with increasing temperature. The curves for the carbon dioxide concentration of the coal samples treated with the compound chemical inhibitors during the oxidation are lower than that of the raw coal sample. At 110 °C, the amount of carbon dioxide produced by the coal sample treated with the compound chemical inhibitor with the compound ratio of 1:2 is lower than that of the raw coal by 150,547 ppm and is 38,314 ppm and 12,633 ppm lower than those of the coal samples treated with the OPC and polyethylene glycol single inhibitors, respectively and is 129,572 ppm, 20,737 ppm and 49,289 ppm lower than those of the coal samples treated with the complex inhibitors with the compound ratios of 1:1, 1:4 and 1:8, respectively. After 110 °C, all of the inhibitors studied here inhibit the production of carbon dioxide. However, the carbon dioxide concentration curve of the compound inhibitor with the compound ratio of 1:2 is always lower than those of proanthocyanidin and polyethylene glycol inhibitors and the other three groups of compound inhibitors.
3.2.2. Mechanism analysis of compound inhibition According to the above experimental results, both OPC and PEG could effectively suppress coal spontaneous combustion. However, the starting temperature of coal spontaneous combustion inhibition and the ability of preventing coal spontaneous combustion were different. The inhibiting effect of composite inhibitors was more obvious than that of single ones. According to results and discussion, the combination of the two has the effect of synergistic inhibition for the three reasons as below. (1) PEG is in the state of viscous liquid. When combined with OPC powder, it can isolate oxygen, improve the disadvantage of PEG’s easy oxidation, and enhance the chemical inhibition of OPC. At the same time, PEG also has the physical resistance effect of moisturizing and cooling, which can further improve the synergistic inhibition effect. The physical state of PEG and OPC are shown in Fig. 19. (2) PEG is rich in hydroxyl groups, which can be associated with OPC to synthesize larger hydrogen proton donors and thus improve the antioxidant activity of OPC. At the same time, with a good solubility in water, PEG can be well dissolved with OPC, so it can not only be used as a solvent or phase transfer agent, but also can act as resistance reaction catalyst [36] in the reaction process, further
(3) Oxygen consumption rate Fig. 18 shows the curves for the dependence of the oxygen 8
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Fig. 20. Schematic diagram of inhibition mechanism.
polyethylene glycol and proanthocyanidin acting as chemical inhibitors provide hydrogen for the peroxide free radical reaction in the coal to form hydrogen peroxide that readily decomposes into alcohol and water under heating. Polyethylene glycol and proanthocyanidin contain a large number of hydroxyl groups that can etherify with alcohols to form stable ether bonds. In addition, the structure in polyethylene glycol and proanthocyanidin can also associate with the hydroxyl groups in the coal to form hydrogen bonds, thus reducing the activity of the hydroxyl groups. To summarize, polyethylene glycol and proanthocyanidins decrease the oxidation rate of the hydroxyl groups on the coal surface, inhibit the oxidation activity of the coal, and reduce its tendency for spontaneous combustion. The chemical inhibition mechanism of PEG and OPC is illustrated in Fig. 20.
Fig. 19. PEG and OPC.
improving the synergistic inhibition effect. (3) According to the chain reaction process of free radicals in the process of coal spontaneous combustion, combined with experimental result and discussions, it can be inferred that EPG and OPC, as chemical inhibitors, can react with the intermediate products in the process of spontaneous combustion to generate relatively stable ether bonds. The analysis is as follows: Previous work in the field has shown [37] that during the action of the coal oxygenation, the oxygen molecule first adsorbs chemically on the high-activity groups on the coal surface, and a chemical bond in the oxygen molecule weakens or even breaks off under the action of the intermolecular force to form eOeOe that then reacts with eCH3 and eCH2 in the coal molecular structure to form hydrocarbon free radicals. Molecules then react with these hydrocarbon radicals to produce peroxide radicals that react with hydrogen to produce hydrogenated peroxides that decompose into different radicals under heating. The main reaction process is shown in (1), (2) and (3), where R represents a hydrocarbon radical.
R·+O2 → R− O− O·
(1)
R− O− O·+H·→ R− O− OH
(2)
R− O− OH → R− O·+OH·
(3)
4. Conclusions In this study, lignite was studied, and the properties of proanthocyanidin and polyethylene glycol as chemical inhibitors for preventing coal spontaneous combustion were studied [39]. The inhibitory effects of OPC and polyethylene glycols were analyzed macroscopically, and the inhibition properties of OPC and polyethylene glycols were further analyzed microscopically. The following conclusions were drawn: (1) The programmed temperature and oxidation experiments for the coal samples treated with OPC and polyethylene glycol showed that OPC and polyethylene glycol could effectively increase the crossingpoint temperature of the coal samples, noticeably delay the appearance time of the crossing-point temperature, reduce the production of CO and CO2 and the oxygen consumption rate, and show good inhibition performance. The inhibition effect of proanthocyanidin is better than that of polyethylene glycol. (2) The best inhibition effect was obtained for the mixing ratio of OPC to polyethylene glycol of 1:2. At this time, the crossing-point temperature of the coal sample was 25 °C higher than that of the raw coal, while that of the coal sample treated with the proanthocyanidin was increased by 17.6 °C, and that of the coal sample treated with the polyethylene glycol was increased by 14.3 °C. Similarly, the CO, CO2 gas production and oxygen consumption rate results showed that the best inhibition effect was obtained for the compound ratio of 1:2. At this time, the oxidation reaction rate of coal was effectively inhibited, and good results were obtained for reducing the spontaneous combustion of coal samples. (3) The combination of OPC and PEG has a synergistic inhibitory effect. The hydroxyl group in PEG can combine with OPC to synthesize larger hydrogen proton donors by hydrogen bond, and improve the antioxidant activity. PEG can be well soluble with PEG and catalyze the catalytic reaction. The combination can not only enhance the antioxidant capacity of PEG, but also keep moisture and cool the coal surface. (4) The hydroxyl groups of OPC and PEG can be etherified with the reaction product alcohols in the process of coal spontaneous combustion to form ether bonds with relatively stable chemical properties. In addition, the hydrogen contained in PEG and OPC can also be associated with the hydroxyl in coal to synthesize hydrogen bonds, thus reducing the hydroxyl activity and inerting coal oxidation activity.
Alkyl and oxygen-containing radicals in coal are oxidized to aldehyde groups in the coal oxidation process at an elevated temperature. Some of the aldehyde groups decompose to produce CO. Some of these aldehyde groups then are further oxidized to carboxyl groups. The carboxyl groups are decomposed to produce CO2. The reaction process is shown in (4), (5) and (6).
R− O·+H·→ R− OH
(4)
R− OH + O2 → R′CHO → R′·+CO
(5)
R′CHO + O2 → R′COOH → R″·+ CO2
(6)
During the continuous generation and combination of free radicals, a large amount of heat is released and accumulated, leading to the gradual increase in the coal temperature. When the temperature reaches a certain level, the fatty groups and oxygen-containing functional groups in the coal begin to react with oxygen, triggering coal spontaneous combustion. The molecular formula of OPC is C30H12O6, which contains a large number of phenolic hydroxyl groups in its molecular structure, which are more likely to combine with active radicals and oxides, thereby reducing the active transmitter and blocking the free radical chain reaction [38].The molecular formula of PEG is HOe(CH2eCH2eO)neH, and the end groups are two hydroxyl groups with primary alcohol properties, which are prone to etherify with some structures in the process of coal oxidation to form a stable ether bond structure. Based on the above free radical chain reaction process and the structural characteristics of PEG and OPC, it is inferred that 9
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Acknowledgements
for blocking air leakage in coal mine. Russ J Appl Chem 2014;87:1099–108. [18] Hu XM, Wang DM, Wang SL. Synergistic effects of expandable graphite and dimethyl methyl phosphonate on the mechanical properties, fire behavior, and thermal stability of a polyisocyanurate-polyurethane foam. Int J Min Sci Technol 2013;23:13–20. [19] Wu JM, Yan H, Wang JF, Wu YG. Flame retardant polyurethane elastomer nanocomposite applied to coal mines as air-leak sealant. J Appl Polym Sci 2013;129:3390–5. [20] Lu Y, Shi SL, Wang HQ, Tian ZJ, Ye Q, Niu HY. Thermal characteristics of cement microparticle-stabilized aqueous foam for sealing high-temperature mining fractures. Int J Heat Mass Transf 2019;131:594–603. [21] Lu Y. Laboratory study on the rising temperature of spontaneous combustion in coal stockpiles and a paste foam suppression technique. Energy Fuels 2017;31(7):7290–8. [22] Li JL, Lu W, Xu J. Coal spontaneous combustion prevention and cure with chemical retarder as well as analysis on retarding mechanism. Coal Sci Technol 2012;40:50–3. [23] Yang YL, Yu SJ, Zhang RY, Yang H, Fan XL. Study on new stopping agent of preventing coal spontaneous combustion. J China Coal Soc 1999;24:163–6. [24] Monagas M, Quintanillalópez JE, Gómezcordovés C, Bartolomé B, Lebrón-Aguilar R. MALDI-TOF MS analysis of plant proanthocyanidins. J Pharm Biomed Anal 2010;51:358–72. [25] Fracassetti D, Costa C, Moulay L, Tomás-Barberána FA. Ellagic acid derivatives, ellagitannins, proanthocyanidins and other phenolics, vitamin C and antioxidant capacity of two powder products from camu-camu fruit (Myrciaria dubia). Food Chem 2013;139:578–88. [26] Pasut G, Veronese FM. PEG conjugates in clinical development or use as anticancer agents: an overview. Adv Drug Deliv Rev 2009;61:1177–88. [27] Harris JM, Chess RB. Effect of pegylation on pharmaceuticals. Nat Rev Drug Discovery 2003;2:214–21. [28] Greenwald RB. PEG drugs: an overview. J Control Release 2001;74:159–71. [29] Zhang YT, Liu YR, Shi XQ, Yang CP, Wang WF, Li YQ. Risk evaluation of coal spontaneous combustion on the basis of auto-ignition temperature. Fuel 2018;233:68–76. [30] Zhang YT, Li YQ, Huang Y, Li SS, Wang WF. Characteristics of mass, heat and gaseous products during coal spontaneous combustion using TG/DSC–FTIR technology. J Therm Anal Calorim 2018;131:2963–74. [31] Zhang L, Liu WL, Men DP. Preparation and coking properties of coal maceral concentrates. Int J Min Sci Technol 2014;24:93–8. [32] Dou GL. Research on chemical inhibitors for preventing coal spontaneous combustion. China University of Mining and Technology Press; 2014. [33] Feng J, Li WY, Xie KC. Study on coal structure by Fourier infrared spectroscopy. J China Univ Min Technol 2002;31:362–6. [34] Yu ZB, Jiang GL. Analytical chemistry experiments. Beijing: Chemical Industry Press; 2006. [35] Zhang GS, Xie YM, Gu JM. Infrared spectroscopy analysis of coal microstructure changes. J Chin Acad Coal 2003;28:473–6. [36] Liang J. Study on the organic reactions using PEG as green medium. Hangzhou: Zhejiang University; 2012. [37] Deng J, Zhao JY, Huang AC, Zhang YN, Wang CP, Shu CM. Thermal behavior and microcharacterization analysis of second-oxidized coal. J Therm Anal Calorim 2016;127:1–10. [38] Zhang Y, Wu XX. Advances in procyanidins research. Pharm Clin Traditional Chin Med 2011;27:112–6. [39] Zhang YT, Shi XQ, Li YQ, Liu YR. Characteristics of carbon monoxide production and oxidation kinetics during the decaying process of coal spontaneous combustion. Can J Chem Eng 2018;96(8):1752–61.
The authors appreciate the financial support of project No. 51474017 provided by the National Natural Science Foundation of China and project No. E21724 provided by the Work Safety Key Lab on Prevention and Control of Gas and Roof Disasters for Southern Coal Mines of China, as well as project No. WS2018B03 provided by the State Key Laboratory Cultivation Base for Gas Geology and Gas Control (Henan Polytechnic University). References [1] Liu L, Zhou FB. A comprehensive hazard evaluation system for spontaneous combustion of coal in underground mining. Int J Coal Geol 2010;82:27–36. [2] Niu HY, Deng XL, Li SL, Cai KX, Zhu H, Li F, et al. Experiment study of optimization on prediction index gases of coal spontaneous combustion. J Central South Univ 2016;23(9):2321–8. [3] Xia TQ, Zhou FB, Wang XX, Zhang YF, Li YM, Kang JH, et al. Controlling factors of symbiotic disaster between coal gas and spontaneous combustion in longwall mining gobs. Fuel 2016;182:886–96. [4] Pone JDN, Hein KAA, Stracher GB, Annegarn HJ, Finkleman RB, Blake DR, et al. The spontaneous combustion of coal and its by-products in the Witbank and Sasolburg coalfields of South Africa. Int J Coal Geol 2007;72:124–40. [5] Niu HY, Qiao CL, An JY, Deng J. Experimental study and numerical simulation of spread law for fire on tunnel. J Central South Univ 2015;22(2):701–6. [6] Wang J, Anthony EJ. CO oxidation and the inhibition effects of halogen species in fluidised bed combustion. Combust Theor Model 2009;13:105–19. [7] Liodakis S, Bakirtzis D, Lois E, Gakis D. The effect of (NHX4)2HPO4 and (NH4)2SO4 on the spontaneous ignition properties of Pinus halepensis pine needles. Fire Saf J 2002;37:48l–94. [8] Wang DM, Dou GL, Zhong XX, Xin HH, Qin BT. An experimental approach to selecting chemical inhibitors to retard the spontaneous combustion of coal. Fuel 2014;117:218–23. [9] Xu YL, Wang DM, Wang LY, Zhong XX, Chu TX. Experimental research on inhibition performances of the sand-suspended colloid for coal spontaneous combustion. Saf Sci 2012;50:822–7. [10] Xu YL, Wang LY, Chu TX, Liang DL. Suspension mechanism and application of sandsuspended slurry for coalmine fire prevention. Int J Min Sci Technol 2014;24:649–56. [11] Ren WX, Guo Q, Zuo BZ, Wang ZF. Design and application of device to add powdered gelling agent to pipeline system for fire prevention in coal mines. J Loss Prev Process Ind 2016;41:147–53. [12] Deng J, Xiao Y, Lu JH, Wen H, Jin YF. Application of composite fly ash gel to extinguish outcrop coal fires in China. Nat Hazards 2015;79:881–98. [13] Qin BT, Dou GL, Wang Y, Xin HH, Ma LY, Wang DM. A superabsorbent hydrogel–ascorbic acid composite inhibitor for the suppression of coal oxidation. Fuel 2017;190:129–35. [14] Zhou FB, Shi BB, Liu YK, Song XL, Cheng JW, Hu SY. Coating material of air sealing in coal mine: clay composite slurry (CCS). Appl Clay Sci 2013;80–81:299–304. [15] Hu XM, Wang DM. Enhanced fire behavior of rigid polyurethane foam by intumescent flame retardants. J Appl Polym Sci 2013;129:238–46. [16] Hu XM, Zhao YY, Cheng WM, Wang DM. Synthesis and characterization of phenolurea-formaldehyde foaming resin used to block air leakage in mining. Polym Compos 2014;35:2056–66. [17] Hu XM, Cheng WM, Wang DM. Properties and applications of novel composite foam
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